This invention is generally directed to an apparatus for transporting a recording medium, such as transparent electrophotographic film, which has a conductive layer, from a supply to a take-up roll, and in particular to a transport apparatus in which an electrostatic attractive force is developed between a drive roller and the film. The electrostatic attractive force prevents slip between the surface of the drive roller and the surface of the film. When the drive roller is driven at a constant angular velocity, the film is transported at a constant linear velocity.
In one type of electrophotographic recording process, the recording medium is a film having a polyester base, a transparent conductive layer that overlies the polyester base and a photoconductive layer that overlies the transparent conductive layer. To record on the film, a uniform electrostatic charge is first applied, by means of a charging corona element, to the surface of the photoconductive layer of the film. The uniform charge is then selectively dissipated by exposing the surface to a light image of the pattern to be recorded. The resulting pattern of charges is an electrostatic latent image on the surface of the photoconductive layer which can then be rendered visible, for example, in an attraction toning process, by applying thereto electrostatically charged developer particles which are held to the surface of the photoconductive layer by means of the electrostatic force developed between the developer particles and the charge on the surface of the photoconductive layer. A permanent visible image can be obtained, for example, by using developer particles which can be heat fused to the photoconductive layer, and then subjecting the visible image to a heat fusing step.
It has been found that when electrophotographic film is processed on a reel-to-reel recorder or processor, the conventional techniques for driving the film and the reels have certain shortcomings. Generally, in recording systems in which the recording medium is fed from a supply reel to a take-up reel, drive is applied to the take-up reel or to both the take-up reel and the supply reel. It is also generally required to have the recording medium travel at a constant linear velocity during the recording process, which means that the reels will travel at a variable angular velocity. In one prior art transport system, the recording medium is fed from a supply reel, over a single drive roller member and onto a take-up reel. The drive roller is driven at a constant angular velocity. The periphery of the drive roller is generally made out of hard rubber which is able to develop sufficient frictional force with the recording medium to drive the recording medium without any slip.
In certain electrophotographic film recorders, there is considerable drag on the film as it travels over the recording element and the prior art single roller drive member is unable to develop sufficient frictional force to drive the film without slip. One common approach used in magnetic tape recorders for developing greater frictional force between the drive roller and the tape employs a pinch roller arrangement for driving the tape with a fixed linear velocity. In such recorders, the tape is passed through a nip formed by a free-wheeling roller and a drive roller. The drive roller presses the tape against the free-wheeling roller thereby developing greater frictional force for driving the tape than can be generated by a single drive roller.
One disadvantage of the pinch roller arrangement in an electrophotographic film recorder is that it exerts considerable pressure on the recording surface of the film. Thus, if the pinch roller is located before the recording station so that it pushes film past the recording stations, it is possible for small dirt particles which may exist on the recording surface of the film to be impressed into the recording surface of the film which will prevent the faithful reproduction of images. If the pinch roller is located after the recording station so that film is pulled past the recording station, the pressure on the recording surface may cause damage to the image recorded on the recording surface of the film.